Ch. 21 Potentiometry 1. General principles Reference electrode : Eref Indicator electrode : E ind The potential of cell E cell = E ind + E ref A cell for potentiometric determinations

* The electrode reaction 2. Reference electrode Ideal reference electrode : * accurate potential, * constant, * completely insensitive to the composition of the analyte solution A. Saturated calomel electrode # Commonly used KCl concentration : 0.1M, 1M, saturated(about 4.6M) * Disadvantage of SCE : more dependent on Temp. than 0.1M, 1M (important only in those rare circumstances when substantial Temp. change occur during a measuring) * ESCE = 0.2444V at 25℃ * The electrode reaction

Typical commercial SCE Length : 5 – 15 cm Diameter : 0.5 – 1.0 cm Hg/Hg(I) chloride paste in saturated KCl solution

B. Silver/Silver chloride electrode

3. Liquid-junction potentials Liquid junction potential : develops across the boundary between two electrolyte solution that have different compositions. 1M-HCl solution contact with 0.01-MHCl solution Conc. Sol. to Dilute sol. Flux : concentration difference H+ : fast Cl- : slow ; derive a separation of charge Difference in diffusion rate

4. Indicator electrode A. Metallic indicator electrode Ideal indicator electrode : * responds rapidly and reproducibly to changes in the concentration of an analyte ion(or group of analyte ions) * Metallic, Membrane, ion-sensitive field-effect transistors A. Metallic indicator electrode First kind electrode Second kind Inert redox electrode 1) Electrodes of first kind A pure metal electrode : in direct equilibrium with its cation in the solution

Not widely used for potential determinations. Cause of several reasons 1) Low selectivity, Respond not only to their own cations but also to other more easily reduced cations. Ex.) Copper electrode cannot used for determination of Cu(II) : presence of Ag+ - Potential : f([Ag+]) 2) Many metal electrode(such as Zn, Cd) can be used for neutral or basic solution : dissolve in the presence of acids 3) Metals are easily oxidized – can be used after remove the oxygen 4) Hard metal(such as Fe, Cr, Co, Ni) : low reproducibility

2) Second kind of electrode Metals : not only serve as indicator electrodes for their own cations but also respond to the activities of anions that form sparingly soluble precipitates or stable complexes with such cations. Ex) Potential of silver electrode correlates reproducibility with the activity of chloride ion in a solution saturated with AgCl. * Potential of silver electrode : proportional to pCl ☞ Silver electrode in a saturated AgCl solution : 2nd kind electrode for Cl-.

Mercury : indicator electrode of second kind for the EDTA anion Y4-. When a small amount of HgY 2- is added to a solution containing Y4-, the half-reaction of a Hg electrode Formation constant for HgY2- : very large(6.3*1021) ☞ the concentration of the complex remains essentially constant over a large range of Y4-. where 3) Inert metallic electrode for redox systems(Pt, Pd,Au, C etc.)

B. Membrane electrode The most convenient method to determine pH : measurement of the potential that appears across a thin glass membrane that separates two solutions with different hydrogen ion concentrations. * Sometimes call p-electrode (p-function : pH, pCa, pNO3)

C. The glass electrode for measuring pH Glass indicator electrode, SCE Indicator electrode : thin, pH-sensitive glass membrane sealed onto the end of a heavy-walled glass or plastic tube(small volume of dilute HCl saturated with AgCl) Silver wire : Ag/AgCl reference electrode

Internal reference electrode : the thin glass membrane bulb at the tip of the electrode that respond to pH # The composition and structure of glass membranes Corning 015 glass : Na2O 22%, CaO 6%, SiO2 72%

# Membrane potentials(4 potential develop in a cell) 1) EAg/AgCl 2) SCE 3) Junction potential, Ej across the salt bridge that separates the calomel electrode from the analyte solution. 4) Most important potential : Boundary potential Eb : Varies with the pH of the analyte solution. # Boundary potential Boundary potential is determined by potentials E1, E2, which appear at the two surfaces of the glass membrane. 1 : the interface between the exterior of the glass and the analyte solution 2 : the interface between the internal solution and the interior of the glass.

For a glass pH electrode : H+ activity of the internal solution is held constant * The boundary potential is then a measure of the H+ ion activity.

# The asymmetry potential When a identical solutions and reference electrode are placed on two sides of glass membrane : Boundary potential = 0. ☞ the boundary potential : change gradually with time ☞ Source of asymmetry potential : obscure * difference in strain on the two surfaces of the membrane imparted during manufacture * mechanical abrasion on the outer surface during use * chemical etching of the outer surfaces ☞ To eliminate the bias caused by asymmetry potential : all membrane electrodes must calibrated one or more standard analyte solutions

# The glass electrode potential The potential of glass electrode, Eind 1) the boundary potential 2) the potential of the internal Ag/AgCl electrode 3) the small asymmetry potential, Easy L = Eind + EAg/AgCl + Easy

# The alkaline and acid error In basic solutions, glass electrode respond to concentration of H+, alkali metal ion

# Liquid-membrane electrode Used for determine the polyvalent cation(Ion-exchange membrane) Comparison of a liquid-membrane calcium ion electrode with a glass pH electrode

Diagram of gas sensing probe Gas-sensing probes Membrane composition Reference electrode Selective ion electrode Electrolyte solution Diagram of gas sensing probe

Assume (aHCO3-) = constant

6. Direct potentiometry Equations governing direct potentiometry

The electrode calibration methode Activity vs concentration

Potentionmetric pH measurement with the glass electrode Errors affecting pH measurements 1) The alkaline error 2) The acid error 3) Dehydration 4) Errors in low ionic strength solutions 5) Vibration in junction potential 6) Error in the pH of the standard buffer The operational definition of pH ES : the cell potential(buffer) EU : the cell potential(unknown)

7. Potentiometric titrations 1) Detecting the end point

2) Neutralization titrations # Determining dissociation constants Half-titration point :

7. Potentiometric determination of equilibrium constants